In nuclear pressurized water reactors with pressurized water, boiling water or reactors cooled by liquid metal, the nuclear fuel exists in the form of pellets stacked in clads sealed at their extremities by plugs so as to constitute elementary rods combined into a bundle by means of a frame in order to form nuclear fuel assemblies each constituting a handling unit.
After the reactor has been in operation for a certain period of time, it is stopped so that one part of the assemblies constituting the core is replaced by new assemblies and so that the assemblies remaining inside the core are moved. During these operations, it is customary to mark the assemblies containing one or more leaky rods.
In the current state of the art, the marking of the assemblies is carried out by analysing the release of radioactive substances during an operation known as a "helium leak test". This technique allows for the marking of assemblies containing leaky rods and, in certain cases, quantifying the amount of leakage from the rods they contain. However, this technique does not mention the localization of effectively defective rods inside the assemblies.
In the particular case of pressurized water and boiling water reactors, the means are known on how to detect the defective rods inside an assembly by marking the rods containing water. Indeed, in the case where the rod sheath is ruptured, the differential pressure between the water of the reactor primary system and the inert gas contained in the rod results in water penetrating inside the rod. Water inside the rods can be detected either by studying the transmission of ultrasonic waves in the upper part of the rods normally filled with gas, or by analysing the damping of waves during a propagation along the rod. This last technique is illustrated in the document FR-A-2 573 906.
However, these two techniques for detecting leaky rods inside nuclear fuel assemblies present different drawbacks. Firstly, their application is limited to the case of water reactors. Moreover, the first technique is effective only if the plenum which is normally filled with gas at the upper part of the rods is fully filled with water, which is not always the case. Finally, the second technique becomes practically ineffective when the clad-pellet gap is reduced, i.e. when the irradiation period exceeds a certain threshold.